1. Field of the Invention
The present invention relates to a fluidized bed incinerator that can incinerate sludge containing an N content while suppressing the generation of N2O that is greenhouse gas, and an incinerating method for sludge using the fluidized bed incinerator.
2. Description of Related Art
Since sludge represented by raw sludge contains a large quantity of N contents derived from protein, various kinds of nitrogen oxides are generated by incineration, and are discharged into the atmosphere. Particularly, N2O (nitrous oxide) of these nitrogen oxides exhibits a Green House effect of 310 times as much as CO2. Therefore, the reduction of N2O is particularly strongly required.
Fluidized bed incinerators, which hardly generate dioxin, have been widely used for incineration of the sludge. Generally, incineration has been carried out at about 800° C. However, when the incineration temperature is raised to 850° C., it turns out that the quantity of N2O generated is decreased. This is referred to as a “high temperature incineration method”, which is estimated as a method effective for suppressing N2O.
However, it is necessary to increase the use quantity of auxiliary fuel to 1.4 to 1.6 times as much as that of the conventional technique in order to raise the incineration temperature to 850° C. The increase is not preferable in view of energy saving. In addition, a current situation where fuel cost is raised causes drastic increase in running cost. Thus, the “high temperature incineration method” is effective for suppressing N2O but has problems in practical use.
The problem of the suppression of N2O is generated even in a fluidizing bed combustion boiler using municipal waste as fuel. Then, Patent Document 1 proposes a multistage combustion method of a fluidizing bed combustion boiler. In the multistage combustion method, the air ratio of a fluidized bed is set to 0.9 to 1.0 to suppress the quantities of N2O and NOx generated. Additional fuel and combustion air therefor are supplied at the upper stage to carry out high temperature combustion to decompose N2O at a high temperature. Furthermore, a sufficient quantity of air is blown at the highest stage to carry out perfect combustion.
However, the multistage combustion method of Patent Document 1 requires a large amount of auxiliary fuel in order to supply the additional fuel and the combustion air therefor to the upper stage of the fluidized bed to form a high temperature place, which can decompose N2O. Since the multistage combustion method of Patent Document 1 is related with a boiler, the multistage combustion method can collect the heat quantity of the auxiliary fuel, and the use quantity of the auxiliary fuel does not become a very large problem. However, when the method is applied to a sludge incinerator as it is, the use quantity of the auxiliary fuel becomes a problem, and the method is not always satisfactory in view of the energy saving.    Patent Document 1: Japanese Patent No. 3059995